The present invention disclosed herein relates to an optical communication system, and more particularly, to a measuring device and a method of measuring signal transmission time difference thereof according to signal transmission between two optical signal paths.
As the optical communication technology advances, a new signal modulation scheme is applied to signal transmission. As an example of the modulation scheme, there is a quadrature phase shift keying (QPSK) scheme, a quadrature amplitude modulation (QAM) scheme, or a coherent optical orthogonal frequency division multiplexing (CO-OFDM) scheme. A receiver receiving a signal modulated in these modulation schemes uses a coherent optical receiver or a balanced optical receiver with a passive optical component such as an optical hybrid or an optical delay interferometer in order to extract phase information.
A time taken for two optical signals output from an optical hybrid (or optical delay interferometer) to arrive at an optical detecting device of a balanced optical receiver is determined according to paths of the two optical lines (or optical fibers). At this time, a time difference between the two paths is called a skew, and when a skew exists, a probability of an error occurrence increases in a received signal.
For a signal transmission in a phase modulation scheme, a skew may mainly occur between output optical lines of an optical hybrid of a receiving end, or input optical lines of a balanced optical receiver. It is very important to perform skew measurement on whether a skew corresponding to a data transmission speed necessary to a system can be provided.
To identically set signal transmission time in optical lines is important for a multimode ribbon optical fiber. Typically, the multimode ribbon optical fiber is used to transmit large amounts of data in a short distance. Accordingly, since a plurality of synchronized signals are transmitted in parallel, as the difference of passing times between signals becomes greater, a data transfer rate is further limited.
In order to measure a skew, measurement is made on what difference actually occurs in signal delay or what change in a phase is made between channels when a signal modulated in a sinusoidal form is transmitted and then received.
In order to directly measure a time difference in signal delay, a signal transmitted through each optical fiber channel is measured through an oscilloscope. At this time, a time difference between measured two signals is measured. However, it is intuitively expected that since a great part of two waveforms overlap when a skew is small, it is difficult to measure a time difference between them.
In addition, in order to measure a skew from a phase difference, a phase shift may be measured through comparison with a reference signal. At this time, a skew may be obtained according to a phase shift between two signals by measuring the phase shift. However, in order to measure a small skew, an optical signal modulated in a high frequency or a precise phase measuring device is required. In particular, since a skew needs to be smaller than 5 ps in 100 Gbps optical communications, it is difficult to measure the skew through a phase shift measurement.